1. Field of the Invention
The present invention relates to ranging signal transmission in a wireless communication system. More particularly, the present invention relates to a method and an apparatus for transmitting a ranging signal by considering Fractional Frequency Reuse (FFR) in a wireless communication system.
2. Description of the Related Art
In a wireless communication system, a mobile station obtains uplink timing and STation IDentification (STID) through a ranging procedure. The ranging procedure generally commences when the mobile station transmits a ranging signal. To transmit the ranging signal, it is necessary to know a location of a resource for transmitting the ranging signal.
According to Institute of Electrical and Electronics Engineers (IEEE)802.16m, the location of the resource for transmitting the ranging signal may be obtained as follows. A format of the ranging signal to transmit and a subframe location are acquired based on a SuperFrame Header (SFH), and a subband location is determined by an IDcell value of a cell carrying the current ranging signal. The location of the subband ISB is determined using mod(IDcell,YSB) which is a function of a total number of subbands YSB and IDcell used in the current cell.
In a multi-carrier Orthogonal Frequency Division Multiple Access (OFDMA) system, resources are allocated based on a subchannel including subcarriers. A plurality of users divides and shares the subcarriers, thus obtaining a multi-user diversity gain in the frequency domain. Orthogonal Frequency Division Multiplexing (OFDM)/OFDMA broadband wireless communication system reuses the same frequency in every cell, applies Adaptive Modulation and Coding (AMC) according to a receive signal intensity and interference between neighboring cells, and thus maximizes throughput. However, in a system with a frequency reuse factor of 1, the interference between the neighboring cells is severe in the boundary of the cell or the sector and the yield is lowered. The system is also subject to service outage. To enhance the performance in the cell boundary with the frequency reuse factor of 1, a Fractional Frequency Reuse (FFR) scheme under consideration divides the whole frequency band into a plurality of Frequency Partitions (FPs), adequately deploys the FP areas, limits the power level of some FP areas in each cell, and thus alleviates the same channel interference between the neighboring cells. When a serving cell prefers and uses some FP areas, other neighboring cells control their power level to reduce the interference level below a certain level with respect to the FP areas preferred by the serving cell.
As stated above, the mobile station in the IEEE 802.16m system determines the subband location ISB of the ranging channel based on the function mod(IDcell, YSB) of the total number of subbands YSB and IDcell used in the current cell.
However, the resource location determination for transmitting the ranging signal does not take account of the FFR. In the FFR, the power level of some FP areas of the neighboring cell is limited, whereas the subband of the ranging channel may be selected in the whole frequency band. As a result, the mobile station in the corresponding cell boundary, as transmitting the ranging signal, may cause interference in the neighboring cell.
What is needed is a method and an apparatus for reducing the interference to the neighboring cell when the ranging signal is transmitted in the wireless communication system which supports the FFR.